"Limpy" was the nickname given by the Twelfth Doctor to a Viking who was trained to be a warrior in preparation for a battle with the Mire. The Doctor chose the name "Limpy" after he injured his leg whilst training with a sword.
When a raiding party returned to their village with the Doctor and Clara Oswald as prisoners, "Limpy" was the villager who blew the horn to announce their arrival. Shortly afterwards the Mire, whose leader was posing as Odin, also arrived, and abducted several of the village's warriors, as well Ashildr and Clara. When Ashildr declared war on the Mire, "Limpy" was one of the villagers chosen to train with the Doctor to become a true Viking warrior.
The Mire returned the following day, but were defeated by the villagers thanks to a plan constructed by the Doctor which involved using electric eels to generate electricity, which in turn would be used to play havoc with the Mire's equipment. "Limpy" celebrated with the rest of his village after the Mire retreated. (TV: The Girl Who Died)